No. 918: Cheapside, EC2

London by Day and Night; or, Men and Things in The Great Metropolis – David W. Bartlett, 1851:

Foul language is often characterized as “billingsgate,” but we dare say there are some in America who use the word without understanding its origin. It comes from the great fish-market in London, called Billingsgate. It is the only wholesale fish-market in London, and necessarily is a scene of great confusion. The people who do business in it are a low and dirty class, and at all hours of the day the market is the scene of noise, confusion, and filth. By degrees the people got to calling language which was foul and noisy, “billingsgate.”